This invention relates to an improved form of electronic voltage regulator. A voltage regulator can be found in virtually every piece of electronic equipment. A voltage regulator has an input terminal and a ground terminal for connection to a source of input voltage, and operates to maintain a constant regulated output voltage at an output terminal.
A voltage regulator can be designed either as a positive voltage regulator or a negative voltage regulator. For convenience, this invention will be described as it relates to a positive voltage regulator, although it will be clear to one skilled in the art how to apply the invention to a negative regulator by appropriate reversal of voltage polarities and use of complementary transistor types.
In a positive voltage regulator, the input voltage V.sub.IN must be larger than the desired output voltage V.sub.OUT, by an increment known as the "dropout voltage." If V.sub.IN is too low, the regulator will be unable to hold V.sub.OUT to the desired level. If V.sub.IN should then fall, V.sub.OUT must fall as well.
A low dropout voltage is important, for example, in battery powered equipment where it is desirable to maintain V.sub.OUT at its designed level for as long as possible as the battery voltage falls. In today's low dropout voltage regulators, the dropout voltage can be as low as 500 millivolts.
Quiescent current can be defined as that part of the current drawn from the input which does not appear in the current supplied at the regulated output. Quiescent current flows through the regulator circuitry to the ground terminal. Since the quiescent current does not contribute useful output, efforts are made to keep it low. A large quiescent current reduces the operating life of battery powered equipment. In other applications, a large quiescent current results in high power dissipation and overheating within the regulator. In today's low quiscent current regulators, the quiescent current can be as low as 50 milliamps at 500 milliamps of output current.
FIG. 1 shows a schematic of a low dropout and low quiescent current voltage regulator known in the art, which is described in U.S. Pat. No. 4,613,809 QUIESCENT CURRENT REDUCTION IN LOW DROPOUT REGULATORS, assigned to National Semiconductor Corporation, the teaching of which is incorporated herein by reference.
Referring to FIG. 1, the input voltage V.sub.IN is applied across input terminal 10 and ground terminal 11. The regulated output voltage V.sub.OUT appears at output terminal 12 due to the action of pass transistor 13 controlled by a driver transistor 14. The driver transistor 14 is activated by an error amplifier 16 which compares a reference voltage V.sub.REF 17 with the regulated output voltage V.sub.OUT reduced through a voltage divider of resistors 18 and 19.
Driver transistor 14 is specially constructed with two collectors 8 and 9. The first collector 8 encircles the emitter and base regions in the transistor, but is in turn encircled by the second collector 9. Further details of this special construction can be found in the above referenced patent. In normal operation, the first driver collector 8 conducts the base current from pass transistor 13 through a diode 15 to the regulated output terminal 12. In this way quiescent current is reduced since its major component, the base current of pass transistor 13, is returned to the regulated output terminal 12.
The second driver collector 9 comes into action as the input voltage V.sub.IN drops close to the regulated output voltage V.sub.OUT. Conduction falls in pass transistor 13 and driver transistor 14 as the voltage difference across their terminals diminishes. At a low enough voltage, a transistor is said to be in saturation, because the voltage difference across its terminals is too low to maintain conduction across its junctions. The first driver collector 8 saturates first when it cannot maintain enough forward bias to conduct through diode 15 to the output terminal 12.
The saturation of the first driver collector 8 allows current to reach the outer, encircling second driver collector 9. Conduction will continue through second driver collector 9 since it is connected to ground, which is at a lower potential than the output terminal 12. This continued conduction allows the regulator to operate slightly longer, to a lower V.sub.IN, before dropout occurs and V.sub.OUT must fall as well. Note however, that during this continued conduction, the benefits of quiescent current reduction are lost, since the current is conducted to ground, rather than returned to the output terminal 12.
The point at which saturation of the first collector 8 occurs and quiescent current increases as the second collector 9 begins to function can be analyzed as: EQU V.sub.IN -V.sub.OUT =V.sub.BE 13+V.sub.SAT 14+V.sub.DIODE 15
Where:
V.sub.IN =Input voltage.
V.sub.OUT =Regulated output voltage.
V.sub.BE 13=Base to emitter voltage drop of pass transistor 13.
V.sub.SAT 14=Saturation voltage of driver transistor 14.
V.sub.DIODE 15=Forward bias diode voltage drop of diode 15.
Since diode 15 could be constructed, for example, by a transistor base to emitter junction, both a transistor base to emitter drop or a diode voltage drop will be designated as a V.sub.BE. The circuit of FIG. 1 therefore suffers from increased quiescent current at a V.sub.IN to V.sub.OUT difference of 2 * V.sub.BE +V.sub.SAT.
It is desirable to continue quiescent current reduction to lower levels of V.sub.IN, and to smaller V.sub.IN to V.sub.OUT differences, and to be able to tailor the level at which the path of conduction changes. It is also desirable to achieve this improved operation without being limited to the inherent fabrication characteristics of the specialized, dual collector construction of the driver transistor 14. It is also desirable to be able to provide multiple paths through which the pass transistor base current can be conducted.